1. Field of the Invention
This invention relates to a bumper for attachment to a front of a vehicle such as an automobile.
2. Description of Prior Art
A front bumper of an automobile generally serves to prevent damage of the automobile body when the automobile is traveling at a speed of 10 km/hour or less and collides with an object such as another automobile or a wall.
Shown in FIG. 14 is one example of such a bumper structure which includes a tubular bumper beam 22, an energy absorbing foam material 23 overlying a front wall of the bumper beam 22, and a bumper fascia 21 overlying the foam material 23. The foam material 23 is designed to absorb collision energy and to restore to its original shape even when subjected to repeated collision impacts. Therefore, the maximum strain of the foam material is designed such that the foam material can maintain its restoration force even when subjected to a large collision impact. To this end, the foam material 23 is required to have a large thickness in the front to rear direction and a relatively high hardness.
However, pedestrians are likely to be seriously injured if struck by an automobile having such a bumper. Recently, there is an increasing demand for a vehicle bumper structure which can protect a pedestrian when struck by an automobile. Thus, the energy absorbing foam material is required to be made of a relatively soft material having a low compression modulus, so that the leg impact can be reduced and serious knee injury can be avoided.
However, since the modem cars are designed to maximize energy efficiency, to increase interior (occupant) space and to provide good appearance, the bumper is required to be compact and light in weight.
At present, no bumper structures on the market can satisfy simultaneously the requirements of (1) prevention of damage to the vehicle, (2) protection of pedestrians and (3) a compact and light weight structure.
To meet with the above requirements (1) and (2), the thought might occur to use a two-layer structure in which a relatively soft foam material for protecting pedestrians is provided in front of a relatively hard foam material for preventing vehicle damage. In this case, however, the requirement (3) cannot be met. When the dimension of the soft foam material in the front-to-rear direction is reduced, collision with a pedestrian causes “bottoming out” of the foam material and generates a large load to cause injury to the pedestrian. Moreover, with the above two-layer structure, it is difficult to maintain the performance of the soft foam material, because collision of the vehicle against a wall or another vehicle will subject the soft foam material to a strain which is beyond the maximum strain thereof so that the soft foam material cannot restore to its original shape. Therefore, when the bumper undergoes a high impact collision, it is necessary to replace the soft foam.
JP-A-H11-208389 discloses a bumper for an automobile which includes a collision energy absorber disposed between a front part of a bumper beam and a bumper fascia. The energy absorber has a lower layer and an upper layer provided on the lower layer and composed of a row of a number of spaced apart blocks arranged with suitable spacing across the width of the vehicle. JP-A-H11-208389 describes that in case of collision with a leg of a pedestrian, the leg is advanced into the space between two adjacent blocks by deflection deformation thereof in the direction of the car width so that reaction force is suppressed and the collision energy is absorbed by only the lower layer. In the case of collision with a wall or another automobile, both the upper and lower layers are compression deformed in the front to rear direction. In practice, however, it is difficult with the above bumper structure to satisfy the above requirements (1), (2) and (3) at the same time. More specifically, (a) a pedestrian's leg is not always received in the space between two adjacent blocks, (b) therefore, the blocks must be thin and/or soft in order to be deformed laterally and to properly receive a pedestrian3 s leg therebetween, (c) the upper layer is apt to be broken, and (d) the dimension of the energy absorber in the front-to-rear direction must be increased to prevent damage to the vehicle so that a compact structure cannot be attained.